The present invention relates to a method and apparatus for producing a magnetic recording medium, in which, while a reactive gas is supplied to a sheet-shaped plasma stream formed in a vacuum chamber, a web-like substrate, which is a constitutional element of a magnetic recording medium, is made to run in the vicinity of the plasma stream so that a thin film of predetermined components is formed on a plasma stream side surface of the substrate by gas-phase reaction of the reactive gas on the plasma stream.
For example, magnetic tape, which is one type of magnetic recording medium, can be generally classified into a magnetic tape having a magnetic layer formed by a coating method and a magnetic tape having a magnetic layer formed by a vapor deposition method.
A magnetic layer formed by vapor deposition is capable of high recording density, and thus a high image quality, but its running durability is low compared with a magnetic layer formed by a coating method.
Therefore, in order to improve the running durability of a magnetic layer formed by a vapor deposition method, a technique has been proposed in which a diamond-like carbon film is formed with a thickness ranging from approximately 50 .ANG. to approximately 200 .ANG. on the magnetic layer to act as a protective layer. (See, for example, Japanese Patent Unexamined Publication No. Sho. 61-210518).
As the aforementioned method for forming a diamond-like carbon film, various plasma CVD (Chemical Vapor Deposition) methods using glow discharge and arc discharge have been proposed heretofore. For example, various methods using radio frequency glow discharge (see Japanese Patent Unexamined Publication No. Sho-63-279426) have been proposed as the method using glow discharge, and various methods using DC arc discharge have been proposed as the method using arc discharge.
In the case where radio frequency glow discharge is used, however, the electron density of the plasma is low so that the deposition rate is low. There thus arises a problem that the production efficiency is low.
Further, a reaction tube must be shaped circularly because of the shape of an induction coil used for generating radio frequency glow discharge. This is unsuitable in the case where the formation of a uniform film over a predetermined width is required, as in the case where the film is formed on a web-like substrate.
Further, in the case where the film-forming process is continued for a long time because of the use of such a reaction tube, the film accumulated in the reaction tube is peeled and mixed as flakes into the film-forming portion of the substrate. For this reason, there may arise defects such as pinholes or the like whereby the film is rendered locally discontinuous, leading to signal dropout.
Further, there has been a risk that film deposited at the outlet of the reaction tube may damage the surface of the substrate.
On the other hand, the method using DC arc discharge is suitable for high deposition rate because the electron density of the plasma is high compared with the case using glow discharge. Further, the method is suitable for the formation of a large-area film because the plasma stream can be shaped into a sheet-shaped configuration by a magnetic field. Accordingly, the method is advantageous also in the case where the formation of a uniform film over a predetermined width is required, as in the case where the film is formed on a web-like substrate.
Therefore, in order to best take advantage of the aforementioned merits of DC arc discharge for a process of forming a diamond-like carbon film, there has been proposed a vacuum film-forming apparatus which is configured by combining a plasma stream generating device for forming a sheet-shaped plasma stream and a web transporting device for running a web-like substrate in the vicinity of the plasma stream.
FIG. 4 shows a conventional example of the aforementioned vacuum film-forming apparatus 1. The apparatus 1 includes a vacuum chamber 3 having a vacuum pump 2 for providing a vacuum atmosphere for the film-forming process, a plasma stream generating device (plasma gun) 5 for forming a sheet-shaped plasma stream 4 in the vacuum chamber 3, a gas supply 6 for supplying a reactive gas to the plasma stream 4, and a substrate transporting device 8 for causing the substrate 7 run in the vacuum chamber 3 in such a manner that a surface (film-forming surface) of the web-like substrate 7 is arranged opposite the plasma stream 4.
The aforementioned substrate transporting device 8 includes an un-winder 10 for delivering the substrate 7 from its wound form, a film-forming drum 11 which delivers the film-forming surface of the substrate from the un-winder 10 while maintained in a state in which the substrate 7 is arranged opposite the plasma stream 4, a take-up device 12 for taking up the processed substrate 7, and electrode rolls arranged in the vicinity of the film-forming drum 11 so as to be pressed by the substrate 7. A DC electric source 14 is connected to the electrode rolls 13 as a bias electric source so as to apply a voltage for accelerating the ionized reactive gas.
However, the electron temperature of plasma generated by DC arc discharge is low so that it may be impossible to ionize and excite the reactive gas sufficiently. Therefore, in the conventional case, as shown in FIG. 4, a bias voltage is applied through the electrode rolls 13 so that the ionization/excitation of the reactive gas is activated.
As shown in FIG. 4, in the case where the bias voltage is a DC voltage, however, arc discharge occurs in the surface of the substrate during the film-forming process because of the application of the bias voltage, as a result of which the quality of the resulting film may be low if, for example, an electrically insulating film such as a diamond-like carbon film is to be formed.
Therefore, it may be thought that the lowering of the quality caused by the occurrence of arc discharge could be avoided by using a radio frequency voltage as a bias voltage instead of the DC voltage. As shown in FIG. 4, in the structure in which the bias voltage is applied through the electrode rolls 13, however, the radio frequency voltage acting on the film-forming surface of the substrate 7 cannot be made uniform. There then arises another problem that the variation of the film thickness distribution is widened because of the variation of the intensity of electric field caused by the variation of the radio frequency voltage.